Indirectly heated cathode



May 30, 1961 H. J. DE WEIJER ETA; 2,985,946

INDIRECTLY HEATED cATHoDE Filed May 9, 1956 'f atent hce Patented May 30, 1961 INDIRECTLY HEATED CATHODE Henricus Johannes De Weijer and Constantius Johannes Henricus Heynen, Eindhoven, Netherlands, assignors, by mesne assignments, to North American Philips Company, Inc., New York, N.Y., a `corporation of Deiaware Filed May 9, 1956, Ser. No. 533,848

Claims priority, application Netherlands May 10, 1955 4 Claims. (Cl. 29--25.14)

This invention relates to indirectly heated cathodes in the form of concentric cylinders electrically insulated from each other and to an improved method for making such cathodes. Cathodes constructed according to the invention are particularly useful in reducing hum interference and microphonics due to heater vibrations in very sensitive tubes.

The cathode consists of a hollow tubular heater shield surrounded by a cathode sleeve from which the heater shield is insulated by a thin layer of dense insulating material. The concentric body thus formed is compacted into an inseparable structure in order to reduce the wall thickness so as to improve heat transfer, to produce a more uniform structure, and to reduce the overall diameter of the cathode to dimensions which are appropriate in the small vacuum tubes of today.

The novel method by which the cathode is formed consists, broadly, in taking a pair of hollow metal tubes, one of which has a larger diameter than the other, coating either the outer surface of the smaller tube or the inner surface of the larger tube with a layer of dense insulating material, telescoping the tubes together, and drawing the resulting concentric body to reduce its wall thickness and overall diameter.

The invention will be described with reference to the accompanying drawings in which:

Fig. 1 shows a cross sectional view of a cathode constructed according to the invention; and

Fig. 2 shows one step in the method of producing cathode of Fig. l.

The cathode in Fig. 1 consists of an outer cathode sleeve 1 and a concentric inner sleeve 2 insulated from each other by a layer of dense insulating material 3. A heater 4 is disposed within the heater shield 2 in the customary manner. The enlarged portion of the cathode indicated by reference character 5 is a rib which is commonly found in cathodes and is provided for the purpose of locating a cathode in the usual mounting structures in a vacuum tube (not shown). A cathode lead 6 is connected to one end of the cathode sleeve 1 and a lead 7 is connected to the heater shield 2 in order that the heater shield may be separately grounded or connected to such voltage source as may be available after a tube utilizing the cathode has been constructed.

In making a cathode of the type shown in Fig. l a metal cylinder is selected of some suitably ductile metal, such as nickel for example, having a diameter and wall thickness larger than the desired dimensions of the heater shield 2. For example the cylinder selected may have a diameter of 4.3 mm. and a wall thickness of 150 microns, although these dimensions are not to be considered as limiting the invention. A second ductile metal cylinder having a larger diameter and greater wall thickness than the rst is selected to become the cathode sleeve l. For example the diameter and wall thickness of the second cylinder may be 5.4 mm. and 200 microns respectively.

Either the outer cylindrical surface of the smaller cylinder or the inner cylindrical surface of the larger cylinder is then coated with a layer of dense or compact insulating material. I have found that the most satisfactory way of producing the layer of insulating material is by means of a cataphoretic or settling down process using aluminum oxide or magnesium oxide as the insulating material. However the invention is not limited to a cataphoretic or settling down process or to aluminum or magnesium oxide but is satisfied by any process and material which will produce a uniformly dense layer on the cylindrical surface. The particles of the insulating material should preferably be as small as possible since the final wall thickness of a completed cathode is limited partly by the size of the particles in the insulating layer.

After the insulating material is applied to one of the two metal cylinders, the cylinders are telescoped together. At this point it is desirable to degas the structure in order to prevent the occurrence of gas bubbles in the insulating layer during the subsequent drawing operation. In order to degas the structure, a gas-tight connection is formed between the outer cylinder and inner cylinder at one end. This may conveniently be done by soldering the outer cylinder to the inner cylinder with silver solder to form a unitary concentric body. This concentric body is then placed in a chamber which is evacuated and the body is heated by induction heating to drive oil gas. Preferably this heating should begin at the closed end of the concentric body and proceed longitudinally to the other end in order that no gas will be trapped. A ring of silver solder may be provided adjacent the other end to be heated by the induction heating apparatus in the iinal step of degassing so that this solder ring will melt and seal the other end of the concentric body to prevent the gas from reentering after the concentric body is removed from the chamber.

The concentric body is then slipped into a mandrel 8 as shown in Fig. 2 and the mandrel, together with the concentric body, is then drawn through a die 9 which serves to reduce the overall Wall thickness of the concentric body. It is desirable to deform one end of the concentric body indicated by reference character 10 to allow it to enter the opening in die 9 without jamming. For example, the end 10 of the concentric body may be hammered into place.

In the event that the wall thickness cannot be sutiiciently reduced by passing the concentric body through a single die, the concentric body may be passed through several dies each of which may give a reductionin diameter of from 1% to 5%. If the Wall thickness has still not been reduced suliciently, the concentric body may be removed from the first mandrel, placed on a second mandrel, and passed through a second series of successively smaller dies. Before being placed on the second mandrel, the concentric body may be annealed. The reason for placing the concentric body on a second and smaller mandrel is that the internal diameter of the concentric body is reduced in the drawing process, although it is highly desirable that the mandrel itself not be reduced in diameter during the drawing process. Therefore the mandrel 8 and the die 9 should be of material which is much harder than the ductile material out of which the cathode sleeve l and the heater shield 2 are made, It may be necessary to roll the concentric body after a drawing operation in order to remove it from the mandrel 8, to which it has a tendency to adhere because of pressure exerted during the drawing.

After the concentric body has been reduced to the desired size it must be cut into suitable lengths. It is common for example to start with metal cylinders 1 and 2 which are from 30 centimeters to l meter in length and which are drawn on a mandrel 8 which may have a length of from 3 to 5 meters. A typical cathode formed as described and starting with cylinders having the'dimcnsions given above may be reduced to an overall order to the hardness of the two metals.

diameter of 850 microns and a total wall thickness of 70 microns of which the cathode sleeve 1 has a diameter of 30 microns and the insulating layer 3 and the heater shield -2 each have a wall thickness of 20 microns. These dimensions are substantially the same as the dimensions of an ordinary cathode constructed from a single piece of metal tubing. However it is possible using the process of the invention to manufacture cathcdes having an outer diameter of about 60() microns and a total wall thickness of 50 microns.

After the concentric body has been cut into the customary lengths for vacuum tube cathodes the cathode sleeve 1 and the insulating layer 3 are [removed from a short length at one end as shown in Fig. l so that the lead wire 7 may be attached to the heater shield Z as indicated. Any air which enters the compressed insulating material after the concentric body has been cut into short lengths may be removed during the usual degassing process after a tube has been built using the cathode.

lt will be noted that in the example given above the wall thickness of the cathode sleeve 1 is greater than the wall thickness of the heater' shield 2. This is desirable in order to prevent rupture of the cathode sleeve 1 during the drawing process, although it is not absolutely essential to the satisfactory formation of a concentric cathode structure. The addition of an electron emissive coating to the outer surface of cathode sleeve 1 and the formation of rib may be carried out after the concentric body has been cut into short lengths. It is a feature of the invention that the compression of the wall of the concentric body produces an inseparable concentric structure which may be handled substantially as if it were only a single piece of metal tubing, since there is no danger of the heater shields 2 falling out of the cathode sleeve 1 under reasonable handling of the concentric structure.

The invention is not limited to the production of concentric cathode structures having only two metal tubes; three or more tubes may be drawn together in the same way. Nor it is it necessary that nickel be used for the cathode sleeve 1 and the heater shield 2; different materials may be used for these elements provided that the initial wall thickness before drawing be selected in inverse Still further modifications may occur to those skilled in the art, and the invention is to be limited only by the following claims.

What is claimed is:

l. The method of making an indirectly heated cathode structure comprising an outer metallic cathode sleeve having an inner cylindrical surface, and an inner heater shield having an outer cylindrical surface located substantially concentrically within said cathode sleeve, said method comprising the stepsv of depositing a compact layer of insulating material on one of said cylindrical surfaces; telcscoping said heater shield within said cathode sleeve; forming a substantially gas tight connection between said heater shield and said cathode sleeve at one end thereof to form a unitary concentric body; degassing said unitary concentric body; inserting a mandrel within said concentric body; drawing said mandrel, together with said concentric body, through a die to form said concentric body into an inseparable concentric structure without substantially reducing the diameter of said mandrel, and thereafter removing the mandrel from said structure.

2. The method of making an indirectly heated concentric cathode structure comprising an outer metallic cathode sleeve having an inner cylindrical surface, and an inner heater shield having an outer cylindrical surface located substantially concentrically within said cathode sleeve, said method comprising the steps of cataphoretically depositing on at least one of said cylindrical surfaces a compact layer of insulating material; telescoping said heater shield within said cathode sleeve; forming a gas tight connection between said heater shield and said cathode sleeve at one end thereof to form a concentric body; placing a ring of solder adjacent the other end of said concentric body; heating said concentric body in a vacuum beginning at the sealed end thereof and continuing along said body to drive gas out of said insulating material; melting said solder to seal said other end of said concentric body; inserting a mandrel within said concentric body; drawing said mandrel, together with said concentric body through a die to form said concentric body into an inseparable concentric structure without substantially reducing the diameter of said mandrel, and thereafter removing the mandrel from said structure.

The method of making an indirectly heated cathode structure which comprises an outer metallic cathode sleeve having an inner cylindrical surface, and an inner heater shield having an outer cylindrical surface located substantially concentrically within said cathode sleeve, said method comprising the steps of cataphoretically depositing on at least one of said cylindrical surfaces a layer of compact insulating material; telescoping said heater shield within said cathode sleeve; forming a substantially gas tight connection between said heater shield and said cathode sleeve at one `end thereof to form a unitary concentric body; degassing said unitary concentric body; forming a second substantially gas tight connection between said heater shield and said cathode sleeve at thc other end thereof; inserting a mandrel within said concentric body; deforming one end of said concentric body to allow the deformed end of said concentric body and said mandrel to enter a die; drawing said mandrel, together with said concentric body, through a series of dies of decreasing diameter t0 compact the wall of said concentric body into an inseparable concentric structure without substantially reducing the diameter of said mandrel, and thereafter removing said mandrel from said structure.

4. The method of making an indirectly heated cathode structure which comprises an outer metallic cathode sleeve having an inner cylindrical surface, and an inner heater shield having an outer cylindrical surface located substantially concentrically within said cathode sleeve, said method comprising the steps of cataphoretically depositing on at least one of said cylindrical surfaces a compact layer of insulating material telescoping said heater shield within said cathode sleeve; soldering one end of said cathode sleeve to said heater shield to form a substantially gas tight connection therebetween and to form a unitary concentric body; placing a ring of solder adjacent the other end of said cathode sleeve; placing said concentric body in an evacuated chamber; heating said concentric body starting at the closed end thereof and progressing to said other end to drive gas from thc region between said heater Shield and said cathode saceve. melting said solder ring onto said other end of said concentric body to form a second substantially gas tight connection between said heater Vshield and said cathode sleeve; removing said concentric body from said evacuated chamber; placing said concentric body upon a mandrel; drawing said mandrel, together with said concentric body, through a series of dies decreasing diameter without substantially reducing the diameter of said mandrel; rolling said mandrel and concentric body to loosen said concentric body from said mandrel; placing said concentric body upon a second mandrel of smaller diameter; drawing said second mandrel, together with said concentric body through a second series of dies of successively smaller diameter to reduce the wall thickness of said concentric body and to compact said concentric body into an inseparable concentric structure without substantially reducing the diameter of said second mandrel, and thereafter removing said second mandrel from said structure.

(References on following page) UNITED STATES PATENTS Corey Aug. 8, 1916 Martin June 1, 1920 5 Hendry Aug. 22, 1922 Everett Jan. 6, 1926 Schumacher Jan. 29, 1929 Myers July 9, 1929 Bohm Feb. 14, 1933 10 6 Kobberup et al. Aug. 24, 1937 Bakarian Ian. 2, 1945 Brace et a1. Mar. 7, 1950 Parker Jan. 8, 1952 Espersen Dec. 22, 1953 Heine et al. Nov. 15, 1955 FOREIGN PATENTS France May 2, 1949 Great Britain Oct. 4, 1888 UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No 2,985,946 May 30, 1961 Henricus Johannes De Weijer et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the saidxLetters Patent. should read as corrected below Column 3, line 40, strike out "it" first occurrence; column 4, line 22, strike out "compacto" and insert the same before "layer", in line 2l, same column.

Signed and sealedthis 14th day of November 1961.

C SEA L) Attest:

ERNEST W. SW1DER Attesting Officer DAVID L. LADD Commissioner of Patents USCOMM-DC 

